This invention relates to methods and apparatus for combining signals. In particular, the invention relates to mixers, modulators and demodulators.
FIG. 1 shows a known mixer 100. A local oscillator signal is applied to port 110. Two further ports are provided: an RF port 118 and an IF port 112. The circuit can perform frequency upconversion or downconversion. In the case of upconversion the IF port 112 is used as a low frequency input and the RF port 118 is used as a high frequency output: if the input is a baseband signal the upconversion process is often referred to as xe2x80x9cmodulationxe2x80x9d and the mixer is referred to as a xe2x80x9cmodulatorxe2x80x9d. In the case of downconversion the RF port 118 is used as a high-frequency input and the IF port 112 is used as a low-frequency output: if the output is a baseband signal the process is often referred to as xe2x80x9cdemodulationxe2x80x9d and the mixer is referred to as a xe2x80x9cdemodulatorxe2x80x9d. The local oscillator signal is coupled, by transformer 114 to the circuit containing diodes 116. The local oscillator waveform controls the switching of diodes 116 and xe2x80x9cchopsxe2x80x9d the input signal, thereby introducing a high frequency component if the input were a low frequency (upconversion), or introducing a low frequency component if the input were a high frequency (downconversion). To facilitate the input and output of the appropriate frequency signals the IF port 112 is connected to diodes 116 via a low-pass filter 122 (signified by an inductor) and the RF port is connected to diodes 116 via a high-pass filter 120 (signified by a capacitor). It can be shown mathematically that if the diodes act as perfect switches there is a perfectly linear relationship between both the amplitude and phase of the input and output signals. Imperfections in this switching typically result in non-linearity in both the amplitude and phase transfer characteristics of the mixer, either of which result in the introduction of additional unwanted spectral components at the output. These are generally referred to as intermodulation products.
There are certain problems and limitations with the mixer 100 of FIG. 1.
First, the balance of the signals supplied from the transformer 114 to the diode circuit is only good at low carrier frequencies and this degrades as the local oscillator frequency increases. This is because the port 110 is connected to an unbalanced source, and capacitive coupling between the windings of transformer 114, which causes signal breakthrough even in the absence of any stimulus being applied to the RF/IF port (112 and 118).
Second, intermodulation performance is only good at low carrier frequencies and also degrades as the local oscillator frequency increases. There is also a considerable variation of the intermodulation performance with frequency in a way that is complicated and difficult to predict and, at high frequencies, it is possible for the performance of the mixer to change from good to bad (or vice versa) over a relatively small change of frequency. It is believed that this performance change is due to an interaction of the switching action of the diodes 116 and the imperfect nature of the LO balancing transformer 114. The transformer 114 only works correctly at frequencies where the electrical lengths of the windings are short in comparison with the wavelength of the signals involved, in this case the local oscillator signal. However, the switching action of the diodes gives rise to a series of spectral components at harmonics of the local oscillator frequency and, at high local oscillator frequencies, the winding-lengths of the transformer will no longer be short in relation to the wavelengths of these spectral components.
Ideally, these harmonics are reflected back through the transformer 114 at the LO port and are ultimately absorbed by the source resistance of the local oscillator. In fact, it is usual to place some resistive attenuation between the LO source and the mixer to assist with this absorption when using local oscillators with poorly controlled source impedance. If this is not possible due to the limitations of the transformer, then the harmonics will be re-reflected back to the diodes in an amplitude and phase which may vary as a function of frequency, and in a way that may be detrimental to the switching action of the diodes 116.
In practice, the mixer 100 works effectively up to approximately 200 MHz. Above this there is a gradual deterioration of balance and intermodulation performance. Above about 1 GHz, balance becomes poor and intermodulation performance is heavily frequency dependent.
Third, the load presented to the local oscillator is highly asymmetric and, in effect, is a half-wave rectifier. A bias path must be provided for the rectification current otherwise the diodes cannot conduct and additionally it may be necessary to adjust this bias to obtain a good diode switching action where the signal mark-to-space ratio is near unity. Without this, excessive mixing will occur at the second and higher even order harmonics of the local oscillator signal.
It is an object of the invention to provide apparatus and methods of combining signals which ameliorate at least some of the aforementioned disadvantages.
According to one aspect, the invention provides signal combining apparatus comprising terminal means, switching means, and balun means, wherein the balun means is connected via the switching means to the terminal means and ground, the switching means being such that when a control signal is supplied to the balun means, the switching means alternately connects the balun means to the terminal means and to ground to generate an output signal related to an input signal in the terminal means.
According to another aspect, the invention also provides a method of combining signals using apparatus comprising balun means connected via switching means to terminal means and ground, the method comprising supplying an input signal to the terminal means, and supplying a control signal to the balun means to alternately connect the balun means via the switching means to the terminal means and to ground so that an output signal is generated in the terminal means.
The invention may be used for frequency up-conversion of the input signal, i.e. modulating the input signal onto a higher frequency signal. Alternatively, the invention may be used for frequency down-conversion of the input signal.
Selectively connecting the balun means to ground via the switching means provides that the balun means has a continuous path by which common mode currents may flow into and out of the balun means. This allows the balun means to have a common mode impedance that may be partly inductive without compromising the nature of the discontinuous current flow between the terminal means and the switching means.
The use of the balun means provides that the switching means is supplied with a balanced version of the control signal, The balun means provides that parasitic admittances to ground are more symmetrical, thereby improving the balance of the balanced signal.
The balun means may comprise a portion of transmission line connected to a portion of dummy line. Preferably, the balun means is a Pausey Stub balun. The Pausey Stub balun may comprise a length of coaxial waveguide with one end of the axial conductor being connected to a cylindrical conductor having substantially the same diameter as the outer conductor of the coaxial waveguide portion. The control signal is supplied to the other end of the axial conductor.
In another embodiment, a stripline implementation of the balun means is used. The transmission line portion of the balun means comprises a three conductor laminate with the control signal connected to the middle conductor. The dummy portion of the balun means comprises a section of the transmission line with the middle conductor omitted. The two layers of the dummy portion are connected to each other and to the middle conductor of the transmission line portion. Vias interconnect the outer conductors of each of the transmission line portion and the dummy portion.
Preferably, the outer surface of the transmission line and dummy portions of the balun means have a distributed resistance to ground thereon. The distributed resistance may decrease along either portion as one moves away from the connection between the portions. The distributed resistance may be provided by a resistive film of varying resistance or by distributed discrete resistors spaced along the outside of the portions. The discrete resistors may be spaced regularly or irregularly. The distributed resistance helps to control unwanted reflections of the control signal within the balun.
The switching means may be a diode arrangement. This diode arrangement may be such that it responds to the waveform of the control signal to alternately connect the balun means to the terminal means and to ground. The output signal may be generated by the chopping action imposed by alternately connecting the balun means to the terminal means and to ground by the diodes.